Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, especially eyeglass lenses. Optical materials, especially eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellow index, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Thus, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, a representative method uses an episulfide compound as described in Patent Document 1.
Meanwhile, in order to improve the oxidation resistance, Patent Document 2 proposes adding a thiol compound to an episulfide compound.
It has also been studied to improve the refractive index. Patent Documents 3 and 4 propose a composition containing sulfur, episulfide and thiol.
However, these composition containing thiol have a problem of being clouded when being polymerized and thus cured. These composition are to be used for optical materials. Therefore, if the composition are clouded after being cured, the composition become all defective. This causes a massive loss. Accordingly, a technique for estimating, on a pre-curing stage, whether the composition will be clouded or not after being cured, so that the composition is determined as being good or not has been desired.